Method for regenerating used absorbents derived from treatment of thermal generator fumes

ABSTRACT

Process and device for regeneration of a used absorbent from a desulfurization zone or from the desulfurization of a gas containing sulfur oxides, comprising regeneration simultaneously with filtering of the absorbent, in a reducing atmosphere, wherein partial combustion of a regeneration gas is also carried out upstream from regeneration, the products of the partial combustion being mixed with the used absorbent prior to the regeneration-filtration stage. The absorbent may be, e.g., solid absorbents based on magnesium oxide. The regeneration gas may be hydrogen sulfide and/or a hydrocarbon. For example, H2S can be partially combusted and the products of the partial combustion, including H 2 S, H 2 , SO 2  and sulfur, mixed with the used absorbent prior to the regeneration-filtration stage.

FIELD OF THE INVENTION

The present invention relates to the sphere of combustion and moreparticularly of regeneration of absorbents used for treatment ofcombustion products.

BACKGROUND OF THE INVENTION

French patent FR-2,636,720, filed by the applicant, discloses a boilerwherein desulfurization agents are injected into a specific zone,referred to as desulfurization chamber, interposed between thecombustion chamber and the convective exchange heat recovery zone. Thedesulfurization agents used in this plant are preferablynon-regeneratable calcic absorbents such as lime or limestone, orindustrial residues with a high calcium carbonate content (sugar millfroth, paper mill froth, etc).

Various improvements have been brought to this type of boilers in orderto increase the thermal efficiency thereof while having the highestpossible efficiency for trapping sulfur oxides notably.

An improvement illustrated in French patent FR-2,671,855 consists inusing absorbents referred to as

regeneratable

absorbents that are regenerated in a regenerator located downstream fromthe boiler, after the final dust separator.

This improvement, which retains the advantages of plants using nonregeneratable absorbents, notably within the scope of desulfurization,also allows to substantially limit the amounts of used absorbent to bedumped, which is favourable to the quality of the environment.

Furthermore, the significant decrease in the amounts of used absorbentto be eliminated allows to envisage inerting treatments atnon-prohibitive costs.

In the plant described above, it is suggested that regeneration of theabsorbent can be carried out by means of a fluidized bed or possibly ofa rotary furnace.

An improvement illustrated in French patent FR-2,730,424 consists incarrying out regeneration simultaneously with filtration of the usedabsorbent in a single reactor.

The processes described above propose using as a regeneration gas ahydrogen-containing or hydrocarbon-containing compound with a totalcarbon number below 10, such as hydrogen, methane, ethane, propane,isobutane and/or a mixture of said gases. Hydrogen is the most suitableregeneration gas because it induces the lowest coking of absorbent.However, supplying hydrogen to an industrial site, a refinery forexample, can pose problems. In fact, hydrogen is not always available insufficient amount in refineries, in particular when substantialconversion and hydrodesulfurization operations are carried out.

Furthermore, the aforementioned two inventions, which allow regenerationof the absorbent, require specific equipments for the regeneration stagethat are costly and numerous.

The present invention allows to eliminate drawbacks and components ofprior plants while keeping the same regeneration efficiency.

Regeneration of a desulfurization absorbent by means of hydrogen sulfideis also proposed in patent FR-2,587,236. No means for carrying out thereaction is however mentioned in this patent.

SUMMARY OF THE INVENTION

The present invention thus allows to minimize the number of equipmentsrequired for the regeneration stage by proposing partial combustion ofthe regeneration gas upstream from the regeneration process and bycombining the regeneration reaction by means of a combustion means suchas a burner for example, preferably located near to the absorbentregeneration and filtration zone. The proximity of the preheating meansand of the regeneration/filtration zone also allows to limit handlingand transport of corrosive fumes containing hydrogen sulfide forexample.

It also allows to use a regeneration gas that is readily available onthe site.

Furthermore, the very nature of the regeneration gas allows to raise theconversion coefficient of the regeneration reaction, as explainedhereafter.

Besides, no regeneration gas recycling is required, so that all thecomponents linked with said recycling can be suppressed.

The object of the present invention thus is a process intended forregeneration of a used absorbent from a desulfurization zone or of anygas containing sulfur oxides, said regeneration being carried outsimultaneously with filtering of said absorbent in a reducingatmosphere, wherein partial combustion of a regeneration gas is carriedout upstream from said regeneration and the products from said partialcombustion are mixed with the used absorbent prior to theregeneration-filtration stage.

According to another feature, the process consists in mixing an additiveregeneration gas during the regeneration-filtration stage.

Said regeneration gas preferably comprises hydrogen sulfide.

It advantageously comprises a hydrocarbon, alone or with hydrogensulfide. Incorporating a hydrocarbon fraction to the regeneration gasallows to substantially increase the hydrogen release during partialoxidation of said gas and thus advantageously favours regeneration ofthe absorbent. The hydrocarbon used is methane for example. It is alsopossible, according to the invention, to use a hydrocarbon gas such asmethane at least a fraction of which is previously partly oxidized priorto contacting with the used absorbent, and which will thus generate aCO/H₂/CO₂/H₂O mixture that is less coking for the absorbent than thehydrocarbon alone.

Furthermore, the gases from the regeneration-filtration stage can becooled.

In addition, the cooled gases can be sent to a Claus plant.

According to a feature of the invention, the regenerated absorbent fromthe regeneration-filtration stage is mixed with a carrier gas prior tobeing sent to a storage unit.

According to another feature of the invention, the regenerated absorbentis mixed with a carrier gas prior to being sent to a desulfurizationzone.

It is well-known to use as regeneration catalysts noble metals of groupVIII, which do not only promote absorption of sulfur dioxide by solidabsorbents based on magnesium oxide for example, but which also catalyzethe absorbent regeneration reaction, such as for example the reductionof magnesium sulfate to magnesium oxide.

Although metals of the platinum group present the advantage of a highcatalytic activity, it is well-known to the man skilled in the art thatsaid activity decreases as a result of the degradation of such acatalyst at high temperatures (above about 900° C.). The most commonlymentioned causes for this performance degradation are the sintering ofthe support and the sintering of the active phase and/or itsencapsulation by the support. It is also well-known that the catalyticactivity of a palladium-based catalyst ranges between 800 and 1000° C.because of the equilibrium as follows:PdO

Pd+½O₂.

It has been found according to the present invention that copper oxideand preferably cerium oxide have a much longer life span than thecatalysts used in the prior art under the temperature conditions of saidregeneration. Surprisingly, it has been found that copper oxide and/orcerium oxide do not only promote absorption of sulfur dioxide by solidabsorbents based on magnesium oxide for example, but that they alsocatalyze the absorbent regeneration reaction such as, for example, thereduction of magnesium sulfate to magnesium oxide.

According to a particular embodiment of the invention, regeneration isthus carried out in the presence of a catalyst.

The catalyst used for said regeneration stage can comprise copper oxideand preferably cerium oxide.

The used absorbent can advantageously be treated prior to being mixedwith the regeneration gas. This treatment can consist of afractionation, carried out for example by means of a cyclone, into atleast two fractions, some of said fractions being rich in catalyst, theothers being poor in catalyst. Said catalyst-rich fractions arepreferably recycled to a desulfurization zone, and said catalyst-poorfractions are either directly sent to the regeneration zone, oradvantageously separated into two streams, one being recycled to adesulfurization zone, the other being sent to the regeneration zone.

Furthermore, the used absorbent can be temporarily stored prior to beingmixed with the regeneration gas.

Another object of the invention is a device intended for regeneration ofa used absorbent from thermal generator fumes treatment, comprising aregeneration means working in a reducing atmosphere by contacting aregeneration gas with the used absorbent, associated with a filtrationmeans, said means comprising an inlet for the used absorbent, an outletfor the gases and an outlet for the regenerated absorbent.

In particular, the device further comprises a means intended for partialcombustion of the regeneration gas and a means for mixing theregeneration gas with the used absorbent, arranged upstream from theused absorbent inlet of the regeneration means.

Besides, the regeneration means can comprise an additional inlet for aregeneration gas.

The device furthermore comprises a means for cooling the gases comingfrom the regeneration means, whose inlet is connected to the gas outlet.

The cooling means can specifically comprise an outlet connected to aninlet of a Claus plant.

Advantageously, the device according to the invention also comprises afiltration means intended to separate the used absorbent from theeffluents at low temperature (T<500° C.) before they enter theregeneration-filtration means, said means being arranged upstream fromthe regeneration means in relation to the direction of flow of theabsorbent.

In particular, the device according to the invention comprises a meansintended for storage of the used absorbent, arranged upstream from theused absorbent inlet in the regeneration means.

The sole FIGURE of the Drawings represents an apparatus used forcarrying out the process of the invention.

Other features, details and advantages of the present invention will beclear from reading the description hereafter, given by way of nonlimitative example, with reference to the accompanying single drawing.

DETAILED DESCRIPTION

In this FIGURE, reference number 1 shows a dust separator allowing tofilter the fumes of a boiler, of a furnace or of any thermal generatorthat specifically burns sulfur-containing liquid or gaseous fuels.

The outlet of such a boiler, as described in patent applicationFR-2,671,855, can thus constitute the inlet of filtering element 1. Theoutlet of any other means generating polluted effluents and equippedwith the same fumes desulfurization means can of course constitute theinlet of element 1.

Dust separator 1, which is however not essential for the smooth runningof the device according to the invention, allows a first separation ofthe fumes coming out of a stack 2 and the used absorbent having absorbedthe sulfur oxides.

The used absorbent is thus discharged gravitationally from filtrationmeans 1 through specific lines 3 and 4. Valves 5 and 6 or any otherequivalent means placed on lines 3 and 4 can allow the used absorbent tobe temporarily stored in element 1. A single used absorbent dischargeline 4 can be provided without departing from the scope of theinvention.

A carrier fluid, fed through lines 22, 23, can be mixed with the usedabsorbent downstream from valves 5 and 6, respectively, in order toensure pneumatic transport thereof. According to an embodiment of theinvention, one of the discharge lines intended for the used absorbent(line 3) is used for recycling part of the absorbent to the boiler. Inany case, a line 4 is provided to convey all or part of the absorbent toa storage hopper 9 used as a buffer to uncouple the operation of theregenerator from that of the thermal generator. This hopper is also usedfor increasing the seal between the (oxidizing) fumes circuit and the(reducing) regeneration gas circuit by preventing passage from one intothe other. The safety of the device is thus increased by preventingspontaneous ignition or explosion risks.

The carrier gas in line(s) 3, 4 can be air or a gas with a low oxygencontent or without oxygen, such as fumes, so as to prevent the presenceof oxygen in hopper 9. This allows to avoid possible combustion orexplosion risks if regeneration gases happen to enter said hopper 9.Said hopper 9 is preferably provided, but it is not obligatory.

The used absorbent leaves hopper 9 through a line 10 that comprises forexample a rotary lock or a valve means 11 and it is sent throughpneumatic transport means to a filter-reactor 12. The gas used forcarrying the used absorbent preferably consists of the regeneration gas,delivered through a line 13 that opens into line 11. This gas, referredto as regeneration gas, whose temperature ranges between 700 and 1500°C., preferably between 900 and 1100° C., is preferably obtained bypartial combustion of H₂S, fed from line 24, in a burner 14 arranged online 13. The sensible heat of the gas in line 13 is sufficient to heatthe used absorbent prior to feeding it into filter-reactor 12. Accordingto another possible embodiment, the used absorbent can be mixed with theregeneration gas at the inlet of the filter-reactor. This is for examplethe case when connection line 13 between regeneration gas partialcombustion device 14 and filter-reactor 12 is reduced to its simplestexpression. This configuration allows to limit thermal wall losses andcontributes to improving the overall energy performance of the plant.Still with a view to reducing thermal losses, the used absorbent and theregeneration gas can also be mixed directly inside the filter-reactor.In this case, the filter-reactor comprises a first mixing zone suppliedby two distinct used absorbent and regeneration gas circuits; this firstmixing zone is followed by the filtering elements proper.

The most part of the used absorbent, carried along by the carrier gas,clings to the filtering elements of filter-reactor 12 where it forms acake. This cake can be periodically unclogged by means of a device 15which momentarily creates a back pressure downstream from the filteringelements by means of a sudden injection of a certain amount ofregeneration gas or of a neutral gas such as nitrogen. Thefilter-reactor preferably consists of several sections, each sectioncomprising one or more filtering elements. With this configuration, thesections can be unclogged one after the other and not simultaneously.Surges are thus minimized, which improves the stability of the plant andfacilitates its operation.

Unclogging causes the cake to fall into the lower part of filter-reactor12 where an accumulation forms.

Fresh regeneration gas can possibly be fed into the lower part offilter-reactor 12 through a specific line 16. A pseudo fluidized bed isthus formed, wherein the regeneration reaction progresses.

The regeneration gas leaves filter-reactor 12 through a line 17. Part ofthe sensible heat of this gas is recovered in a heat exchanger 18arranged on line 17. This heat can for example be used for generatingsteam used on the site. Heat exchanger 18 can be advantageously equippedwith a liquid sulfur recovery means, this liquid sulfur having formedduring the stage of partial combustion of the regeneration gas andduring regeneration of the absorbent. The liquid sulfur is dischargedthrough a line that is not shown in the figure and sent to the circuitof a Claus plant for example.

At the outlet of heat exchanger 18, the regeneration gas can be carriedto a Claus plant through a line 19. Considering its composition, theregeneration gas can be introduced directly into the first sulfurcondenser of the Claus plant (not shown).

The present invention thus allows to use as a regeneration gas hydrogensulfide that is always present in large amounts in refineries, andnormally processed in a Claus plant to be converted to sulfur.

The present invention thus allows to raise the treating capacity of theClaus plant by using part of the hydrogen sulfide intended for the Clausplant for regeneration of the used absorbent.

The regenerated absorbent is discharged in the lower part offilter-reactor 12 through a line 20 that comprises for example a rotarylock and an ejector. A pneumatic carrying gas flows into line 20 via aline 21. The regenerated absorbent can for example be carried to thedesulfurization zone of a boiler or to a temporary storage point.

A beneficial effect linked with the invention is obtained throughpartial combustion of the hydrogen sulfide contained in the regenerationgas: the formation of hydrogen through dissociation, during combustionof H₂S, allows to enrich the regeneration gas in H₂, which allows toraise the conversion coefficient of the regeneration reaction.

Besides, recycling of the regeneration gas is no longer necessary, whichallows to cut out certain constituents such as a draft ventilator. Infact, the gaseous effluent resulting from the absorbent regeneration canbe sent to a Claus plant upstream or downstream from the firstcondenser.

A quantified embodiment example is described hereafter in connectionwith the figure.

A 5094 kg/h flow of used absorbent is extracted from dust separator 1through line 4. This absorbent has a mass sulfation ratio of 58% and atemperature of 180° C. It is sent by pneumatic transport into hopper 9by using 5100 kg/h desulfurized fumes as the carrier fluid.

A 3510 kg/h flow of H₂S is fed through line 24 into burner 14 where itis partly oxidized by a 4000 kg/h air stream. The fumes obtained aredischarged through line 13. They are at a temperature of 1128° C. andcontain 15% H₂S by weight, 0.4% H₂ by weight, 1.2% SO₂ by weight and 29%sulfur by weight. These fumes are mixed with the used absorbentextracted from hopper 9 through line 11 and fed into filter-reactor 12.The inlet temperature is 790° C.

The regenerated absorbent is extracted from filter-reactor 12 throughline 20. This absorbent has a flow rate of 3900 kg/h, a sulfation ratioof 26% and a temperature of 680° C.

The gas used for regeneration is extracted from filter-reactor 12through line 17. It is cooled to 350° C. by exchanger 18. This gas has aflow rate of 8700 kg/h and contains 12% SO₂ by weight, 12.7% H₂S byweight and 25.4% sulfur by weight.

It is well-known that the regeneration gas and the used absorbent mustbe brought to a temperature ranging between 600° C. and 1000° C. toallow the regeneration reaction to take place. In the prior art, heatingis provided by means of an exchanger followed by an oven. According tothe present invention, the regeneration gas is advantageously heated bypartial combustion of hydrogen sulfide, which allows the regenerationgas to be brought to a temperature above 1000° C. The used absorbent isheated by mixing the gas and the absorbent prior to feeding it into theregeneration unit. This heating means thus allows to cut out two costlyequipments of the regeneration circuit: the exchanger and the preheatingoven.

This economic advantage is quite interesting, all the more so because itis not detrimental for the regeneration efficiency or any other workingparameter of the plant.

1. A process for regeneration of a used absorbent from a desulfurizationzone or from the desulfurization of any gas containing sulfur oxides,said regeneration being carried out simultaneously with filtering ofsaid absorbent in a reducing atmosphere, characterized in that itcomprises carrying out partial combustion of a regeneration gas upstreamfrom said regeneration and in that the products of said partialcombustion are mixed with the used absorbent prior to theregeneration-filtration stage, and in that the used absorbent isfractionated, prior to being mixed with the regeneration gas, into atleast two fractions, some of said fractions being rich in catalyst, theothers being poor in catalyst.
 2. A process as claimed in claim 1,characterized in that said regeneration is carried out in afilter-reactor and in that the process further comprises mixingadditional regeneration gas during the regeneration-filtration stage, inthe filter-reactor.
 3. A process as claimed in claim 1, characterized inthat said regeneration gas comprises hydrogen sulfide and/or ahydrocarbon.
 4. A process as claimed in claim 1, characterized in thatthe gases from the regeneration-filtration stage are cooled.
 5. Aprocess as claimed in claim 4, characterized in that the cooled gasesare sent to a Claus plant.
 6. A process as claimed in claim 1,characterized in that the regenerated absorbent from theregeneration-filtration stage is mixed with a carrier gas, then sent toa storage unit.
 7. A process as claimed in claim 1, characterized inthat the regenerated absorbent is mixed with a carrier gas, then sent toa desulfurization zone.
 8. A process as claimed in claim 1,characterized in that said regeneration is carried out in the presenceof a catalyst.
 9. A process as claimed in claim 8, characterized in thatthe catalyst used for said regeneration comprises copper oxide and/orcerium oxide.
 10. A process as claimed in claim 1, characterized in thatsaid catalyst-rich fractions are recycled to a desulfurization zone, andsaid catalyst-poor fractions are directly sent to the regeneration zone.11. A process as claimed in claim 1, characterized in that saidcatalyst-rich fractions are recycled to a desulfurization zone, and saidcatalyst-poor fractions are separated into two streams, one beingrecycled to a desulfurization zone, the other being sent to theregeneration zone.
 12. A process as claimed in claim 1, characterized inthat the used absorbent is temporarily stored prior to being mixed withthe regeneration gas.